Pressurized water-moderated (PWR) and boiling water nuclear (BWR) reactors use fission for producing steam to drive a steam turbine. A metal reactor core and other control apparatus, commonly referred to as reactor internals, are disposed in a metal vessel filled with water. Fission takes place within the reactor core whereby the energy of the fission products is transferred to the water. In a PWR, the heated water is pumped from the reactor vessel through a heat exchanger in which it transfers its heat energy to another circuit of water to form steam to drive a turbine. In a BWR, the steam goes directly from the reactor to the turbine.
In both types of reactors, neutrons, which are a product of the nuclear reaction, embrittle the reactor core walls. Continued embrittlement of the walls may result in excessive sensitivity to fracture. Periodic annealing of the vessel walls can reverse the effects of embrittlement, thus preventing fracture sensitivity and extending the useful life of the vessel.
Proper annealing requires heating the vessel walls for a long period of time at a high temperature. A typical steel reactor vessel requires heating in the range of 650.degree.-850.degree. F. dependent on the chemical composition of the steel. The heating duration ranges from 2-9 days dependent on the temperature and amount of property recovery sought. The temperature of the annealing operation is carefully controlled to prevent excessive vessel stress or over-temperature of the primary shield concrete.
One method for annealing pressurized water-moderated reactor vessels is to raise the primary system water to the required annealing temperature and circulate the heated water through the vessel. As the temperature of the water increases, the pressure increases. Thus, the annealing temperature is limited by design pressure limits of the reactor vessel and primary system. Standard reactors using heated water for annealing are limited by a maximum water temperature of 650.degree. F., which either achieves only limited embrittlement recovery and/or requires excessive time for substantial recovery.
Another method predicates removing all internal components from the reactor pressure vessel and introducing into the vessel an electrical resistance heater to achieve the required annealing temperatures. This method is complex, time consuming and not feasible at storage limited sites due to difficulties associated with the reactor internals removal process. Since, the heating element transfers heat to the vessel by thermal radiation, it must reach temperatures substantially hotter than the target annealing temperature. Other components in close proximity to the heating element with lower temperature limits than the vessel must be protected from this high temperature source.